The invention relates to an indicating instrument, especially in a motor vehicle, having a cover glass that appears dark and, under incident light, rules out any view of a dial arranged at a distance behind it, and having at least one light source arranged behind the dial to transilluminate said dial and therefore render the dial visible through the cover glass.
Indicating instruments of the above type are known under the term xe2x80x9cblack-panel designxe2x80x9d and, to some extent, have entered practical use. The black-panel effect is realized by the transmissivity of the cover glass being reduced to 20% to 40%, in the manner of sunglasses, by means of a color print. In the known indicating instruments, the dial is designed to be light, so that it appears easily visible to the viewer under transmitted-light illumination, in spite of the dark front glass.
It has been shown that, in order to implement a good black-panel effect, the cover glass has to be relatively highly colored, in order that the dial appears invisible when the transmitted-light illumination is switched off. However, since as a rule no incandescent lamps can be used for the illumination, since they produce a great deal of heat, and light-emitting diodes are therefore preferred, there is quite a restriction relating to the available luminous intensity. The aim is therefore to permit the transmitted light to pass from the light source to the viewer with the lowest possible losses. However, this objective is opposed by the desire not to permit the dial to become visible merely as a result of incident light.
The invention is based on the object of configuring an indicating instrument of the type mentioned at the beginning in such a way that a black-panel effect is implemented without undesirably high luminous intensities being needed for the transmitted-light illumination.
According to the invention, this problem is achieved in that, in order to prevent the dial from becoming visible when there is only incident light, the transmission of the dial is matched to the transmission of the cover glass.
As a result of the indicating instrument according to the invention, two parameters are available to bring about the black-panel effect. Matching the dial to the transmission behavior of the cover glass makes it possible to reduce the transmission of the cover glass relatively slightly. However, the incident light which then passes through the cover glass onto the dial does not lead to the dial becoming visible, since the latter, due to the invention, has its transmission matched to the cover glass in such a way that light incident from the outside does not pass back through the cover glass to the viewer. Overall, this matching according to the invention allows a good black-panel effect to be achieved, without unworkable requirements having to be placed on the luminous intensity of the light source.
The black-panel effect can be reinforced in a simple way by the transmission of the cover glass being matched to the visible spectrum of the light source and, for example, by selecting a high transmission for the wavelength of the light source but a low transmission for other wavelengths. As a result, incident light can be kept back from the dial without the transmission of the light originating from the light source being restricted.
The indicating instrument is configured particularly simply if the cover glass has a dark coloration which permits a dial that reflects well toward the cover glass still to appear visible, and therefore has a low transmission, and if the dial is designed to reflect poorly. In the case of such an indicating instrument, under incident light, a relatively large amount of light may strike the dial without it being reflected back to the viewer, since, as a result of its dark coloration, the dial reflects only a little light.
The poor reflection of the dial may be implemented very simply by the dial being designed to be dark.
Alternatively, however, it is also possible for the dial to be designed to reflect poorly as a result of its surface form. To this end, the dial can, for example, be provided with a very highly roughened surface.
A further contribution to reducing the luminous intensity is made by the cover glass and the dial having a visible-wavelength selective coloration.
In the case of a dial with a negative display, it is possible for the light from the light source that reaches the non-translucent areas to be used for the transillumination of the translucent areas as well if, on its side facing the light source, the dial has white printing in areas that are not to be transilluminated. By this means, the light which reaches the non-translucent areas is reflected and, following renewed reflection, can reach the translucent areas.
Instead of restricting the reflective capacity of the dial, according to another configuration of the invention, it is also possible to provide for the cover glass to be provided with a polarization filter, and for the dial to be designed to be highly reflective. In such a design, use is made of the effect that light passing through a polarization filter cannot pass back through the polarization filter following a reflection.
The light source used for the indicating instrument can be a conventional cold-cathode tube. This has a high luminous intensity and, by means of appropriate curvature, can extend over the area to be illuminated.
Cold-cathode tubes have only a low luminous intensity at low temperatures and can be dimmed only with difficulty. This can be rectified if, according to another configuration of the invention, provision is made for the light source used to be a large number of individual light-emitting diodes.
A particularly low luminous intensity of the light source is needed if the dial is designed with a positive display.
In an indicating instrument in which the dial is designed with a negative display, optimum illumination can be implemented with a low current consumption by means of light-emitting diodes if a light-emitting diode is situated behind each scale mark and/or behind each number or each indicating symbol, as a light source.